Lubricating oil composition for shock absorber

ABSTRACT

To provide a lubricating oil composition for a shock absorber for preventing wearing a piston rod and a guide bush in a shock absorber, which composition lowers the friction coefficient at the interface between bronze and chromium and reduces wear area of bronze. 
     The lubricating oil composition for a shock absorber contains (A) a base oil composed of a mineral oil and/or a synthetic oil, and (B) a polyhydric alcohol partial ester having a C10 to C20 fatty acid residue, in an amount of 0.05 mass % to 10 mass %, with respect to the total amount of the composition.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition for ashock absorber. More specifically, the present invention relates to alubricating oil composition for a shock absorber mainly used at asliding part between a guide bush and a piston rod in a shock-absorberof a four-wheeled vehicle.

BACKGROUND ART

Lubricating oil for shock absorbers in automobiles is employed mainlyfor damping vibration in order to attain optimum attenuation force andmaintain driving stability. Generally, shock absorbers are disposed inan automobile between the body and the tires and attenuate vibration ofthe car body caused by bumps of a road, jolting generated at quickacceleration or heavy braking, and other motions.

Hitherto, lubricating oils for shock absorbers in automobiles haveexhibited enhanced vibration damping effect through reducing thefriction at a sliding interface between an oil seal and a piston rod, apiston rod and a guide bush, a piston band and a cylinder, etc. in ashock absorber (see, for example, Patent Documents 1 and 2).

Patent Document 3 discloses a lubricating oil composition containing abase oil and at least one species selected from among an alkenylsuccinimide, an acidic phosphite diester, and a perbasic sulfonate,phenate, or salicylate of an alkaline earth metal, for the purposes ofenhancing frictional force at the interface between an oil seal and apiston rod, reducing the friction coefficient between the piston rod andthe guide bush, and suppressing foaming.

Generally, a shock absorber is arranged not in a direction orthogonal tothe road but is slanted from the orthogonal direction, since the slantarrangement provides more excellent riding comfort. Thus, duringexpansion and contraction of a shock absorber, large lateral forceattributed to a generated bending moment is applied to the shockabsorber. In order to facilitate expansion and contraction of the shockabsorber under application of lateral force, a shock absorber oil (shockabsorber fluid: SAF) is required to reduce friction of a bearing (guidebush). Particularly when the guide bush of a shock absorber has worn,oil leakage occurs, thereby failing to gain appropriate attenuationpower, which is problematic.

As described above, the piston rod/guide bush friction has been reducedby use of a phosphorus-containing additive or a fatty acid. However,phosphorus-containing additives generally have poor thermal stability,and fatty acids generally have poor wear resistance.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. Hei5-255683

Patent Document 2: Japanese Patent Application Laid-Open (kokai) No.2000-192067

Patent Document 3: Japanese Patent Application Laid-Open (kokai) No.2009-298886

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Under such circumstances, an object of the present invention is toprovide a lubricating oil composition for a shock absorber that canimprove lubricity between bronze and chromium, which are materialsgenerally used in a guide bush and a piston rod; specifically, thecomposition lowers the friction coefficient therebetween and reduceswear area of bronze, to thereby reduce the friction coefficient betweena piston rod and a guide bush and, furthermore, to prevent wearing ofthe guide bush.

Means for Solving the Problems

The present inventor has conducted extensive studies to develop alubricating oil composition for solving the aforementioned problems, andhas found that the object can be attained by adding a specific amount ofa specific polyhydric alcohol partial ester to a specific base oil. Thepresent invention has been accomplished on the basis of this finding.

Accordingly, the present invention provides the following.

[1] A lubricating oil composition for a shock absorber, comprising:

(A) a base oil composed of a mineral oil and/or a synthetic oil, and;

(B) a polyhydric alcohol partial ester having a C10 to C20 fatty acidresidue, in an amount of 0.05 mass % to 10 mass %, with respect to thetotal amount of the composition.

[2] A lubricating oil composition for a shock absorber as described in[1] above, wherein the component (B) is a tetrahydric alcohol partialester.

[3] A lubricating oil composition for a shock absorber as described in[1] or [2] above, wherein the component (B) is pentaerythritol dioleateand/or pentaerythritol dilaurylate.

[4] A lubricating oil composition for a shock absorber as described inany of [1] to [3] above, which further comprises (C) aphosphorus-containing compound.

Effects of the Invention

According to the present invention, there can be provided a lubricatingoil composition for a shock absorber that can improve lubricity betweenbronze and chromium, which are materials generally used in a guide bushand a piston rod; specifically, the composition lowers the frictioncoefficient therebetween and reduces wear area of bronze, to therebyreduce the friction coefficient between a piston rod and a guide bushand, furthermore, to prevent wearing of the guide bush. Particularly,the lubricating oil composition for a shock absorber of the presentinvention can provide automobile users with excellent riding comfortthrough reduction in friction coefficient. Furthermore, the lubricatingoil composition of the present invention prevents friction in a shockabsorber, to thereby enhance durability of the shock absorber, wherebythe shock absorber can exhibit excellent riding comfort and highdurability.

Modes for Carrying Out the Invention

The lubricating oil composition for a shock absorber of the presentinvention comprises;

(A) a base oil composed of a mineral oil and/or a synthetic oil(hereinafter may be referred to simply as “component (A)”), and;

(B) a polyhydric alcohol partial ester having a C10 to C20 fatty acidresidue (hereinafter may be referred to simply as “component (B)”), inan amount of 0.05 mass % to 10 mass % with respect to the total amountof the composition.

<(A) Base Oil>

The base oil (A) of the lubricating oil composition of the presentinvention is a mineral oil and/or a synthetic oil. No particularlimitation is imposed on the type of the mineral oil and synthetic oil.Examples of the mineral oil include paraffin-based mineral oil,intermediate mineral oil, and naphthene-based mineral oil, which areproduced through a routine refining method such as solvent refining orhydrogenation refining.

Examples of the synthetic oil include polybutene, polyolefins [e.g.,α-olefin (co)polymers], esters (e.g., polyol-esters, dibasic acidesters, and phosphoric acid esters), and ethers (e.g., polyphenylether), alkylbenzenes, and alkylnaphthalenes.

In the present invention, the mineral oil may be used, as the base oil,singly or in combination of two or more species. Also, the synthetic oilmay be used, as the base oil, singly or in combination of two or morespecies. Furthermore, one or more mineral oils may be combined with oneor more synthetic oils.

Among these oils, mineral oils, particularly paraffin-based mineraloils; α-olefin polymers, such as 1-decene oligomers; and mixturesthereof are preferably employed.

The lubricating oil composition of the present invention is mainlyemployed as an automobile shock absorber oil. Thus, the viscosity of thebase oil is preferably within the range of 2 to 20 mm²/s, morepreferably 3 to 15 mm²/s, and still more preferably 4 to 10 mm²/s interms of kinematic viscosity at 40° C.

No particular limitation is imposed on the viscosity index of the baseoil, but it is preferably 95 or higher, more preferably 100 or higher,still more preferably 105 or higher. In the case where a plurality ofbase oils are used in combination, properties of the base oils includingviscosity index mean those of the base oil mixture.

The base oil preferably has a flash point of 150° C. or higher, morepreferably 155° C. or higher. When the flash point of the base oil is150° C. or higher, foaming is suppressed in use of the oil composition,which may enhance riding comfort.

Therefore, it is not preferred to use a base oil to which an excessiveamount of low-viscosity base has been added for the purpose ofenhancement of low-temperature flowability.

In the present invention, the flash point is generally measured throughJIS K2265 (COC method).

No particular limitation is imposed on the component (A) content of thelubricating oil composition of the present invention. For example, thecomponent (A) content is preferably 50 mass % to 99.9 mass %, withrespect to the total amount of the composition, more preferably 70 mass% to 99.8 mass %, still more preferably 80 mass % to 99.7 mass %.

<(B) Polyhydric Alcohol Partial Ester>

Component (B) is a polyhydric alcohol partial ester having a C10 to C20fatty acid residue. As used herein, the term “fatty acid residue” refersto a residue formed through removal of a carboxyl group from a fattyacid.

No particular limitation is imposed on the fatty acid residue, and theresidue may be branched or linear-chain, or may be an unsaturatedaliphatic hydrocarbon residue or a saturated aliphatic hydrocarbonresidue. Specific examples of the fatty acid residue include decyl,lauryl, palmityl, stearyl, and oleyl.

Examples of the polyhydric alcohol include dihydric alcohols, such asethylene glycol, diethylene glycol, propylene glycol, and dipropyleneglycol; trihydric alcohols, such as glycerin, trimethylolethane,trimethylolpropane, trimethylolbutane, 1,3,5-pentanetriol,1,2,4-butanetriol, 1,2,6-hexanetriol; tetrahydric alcohols, such aspentaerythritol, 1,2,3,4-butanetetrol, and sorbitan; and polyhydricalcohols, such as adonitol, arabitol, xylytol, sorbitol, and mannitol.

No particular limitation is imposed on the valency of the polyhydricalcohol, but the number of hydroxyl groups is preferably 3 or more, morepreferably 4.

The polyhydric alcohol preferably has 1 to 3 ester substituted sites,more preferably 2 to 3 ester substituted sites.

The polyhydric alcohol partial ester preferably has 2 or more hydroxylgroups.

Examples of preferred polyhydric alcohol partial esters includepentaerythritol dioleate, pentaerythritol dilaurylate, andpentaerythritol distearylate.

Of these, pentaerythritol dioleate and pentaerythritol dilaurylate areparticularly preferred.

The component (B) content of the lubricating oil composition of thepresent invention is 0.05 mass % to 10 mass %, with respect to the totalamount of the composition, preferably 0.2 mass % to 4 mass %, morepreferably 0.3 mass % to 3 mass %.

<(C) Phosphorus-Containing Compound>

The lubricating oil composition of the present invention preferablycontains (C) a phosphorus-containing compound (hereinafter may bereferred to simply as “component (C)”). The phosphorus-containingcompound exerts synergistically with polyhydric alcohol partial ester(B), to thereby provide considerably enhanced wear resistance.

Examples of the phosphorus-containing compound (C) include phospho-estercompounds such as a phosphate ester, an acidic phosphate monoester aminesalt, and an acidic phosphite diester, and zinc dithiophosphate (ZnDTP).

The lubricating oil composition of the present invention preferablycontains, as a phosphorus-containing compound among them, ZnDTP having aC7 to C12 alkyl group. Examples of the ZnDTP includes compoundsrepresented by the following formula (I)

(wherein each of R¹ and R² represents a C7 to C12 linear-chain,branched, or cyclic alkyl group).

Specific examples of the alkyl group R¹ or R² in formula (I) includeheptyl, isoheptyl, cyclohexylmethyl, octyl, 2-ethylhexyl, isooctyl,cyclooctyl, nonyl, isononyl, 3,5,5-trimethylhexyl, cyclooctylmethyl,decyl, 3,7-dimethyloctyl, 2-propylheptyl, isodecyl, undecyl, dodecyl,2-butyloctyl, and isododecyl. Among them, C7 to C10 alkyl groups aremore preferred.

R¹ and R² may be identical to or different from each other. However,they are preferably the same group, from the viewpoint of easiness ofproduction.

Examples of the phosphate ester compound include an acidic phosphoricacid monoester amine salt formed from an acidic phosphoric acidmonoester having a C1 to C8 alkyl or alkenyl group; e.g., monomethylhydrogenphosphate or monoethyl hydrogenphosphate, and an amine compoundhaving a C8 to C20 alkyl or alkenyl group.

The lubricating oil composition of the present invention preferably hasa component (C) content; i.e., a phosphorus-containing compound content,of 0.3 to 2 mass %, with respect to the total amount of the composition,more preferably 0.5 to 1.5 mass %.

<Other Optional Components>

So long as the object of the present invention is not impaired, theshock absorber oil of the present invention may appropriately contain,as an optional additive, at least one species selected from among anashless detergent-dispersant, a metallic detergent, a lubricationimprover, an antioxidant, a rust preventive, a metal deactivator, aviscosity index improver, a pour point depressant, and a defoamingagent. No particular limitation is imposed on the optional components,and the amounts of these components are preferably 0.1 to 20 mass %,with respect to the total amount of the composition, more preferably 0.3to 10 mass %, still more preferably 0.3 to 5 mass %.

Examples of the ashless detergent-dispersant include divalentcarboxamides, such as a succinimide, a boron-containing succinimide, abenzylamine, a boron-containing benzylamine, and succinic acid. Examplesof the metallic detergent include a neutral metal sulfonate, a neutralmetal phenate, a neutral metal salicylate, a neutral metal phosphonate,a basic sulfonate, a basic phenate, a basic salicylate, a perbasicsulfonate, a perbasic salicylate, and a perbasic phosphonate.

Examples of the type of the lubrication improver include an extremepressure agent, an antiwear agent, and an oiliness agent, and examplesof the material of the lubrication improver include organometalliccompounds, such as zinc dithiocarbamate (ZnDTC), oxysulfidomolybdenumorganophosphorodithioate (MoDTP), and oxysulfidomolybdenumdithiocarbamate (MoDTC).

Examples of the sulfur-containing extreme pressure agent includesulfurized oils, sulfurized fatty acids, sulfurized esters, sulfurizedolefins, dihydrocarbyl polysulfide, thiadiazole compounds,alkylthiocarbamoyl compounds, triazine compounds, thioterpene compounds,and dialkylthio dipropionate compounds.

Examples of the oiliness agent include aliphatic saturated andunsaturated monocarboxylic acids, such as stearic acid and oleic acid;polymerized fatty acids, such as dimer acid and hydrogenated dimer acid;hydroxy fatty acids, such as ricinoleic acid and 12-hydroxystearic acid;aliphatic saturated and unsaturated monoalcohols, such as lauryl alcoholand oleyl alcohol; aliphatic saturated and unsaturated monoamines, suchas stearylamine and oleylamine; and aliphatic saturated and unsaturatedmonocarboxamides, such as lauriamide and oleamide.

Examples of the antioxidant include polycyclic phenol-basedantioxidants, such as 4,4′-methylenebis(2,6-di-tert-butylphenol) and2,2′-methylenebis(4-ethyl-6-tert-butylphenol); amine-based antioxidants,such as monoalkyldiphenylamine compounds; e.g., monooctyldiphenylamineand monononyldiphenylamine, dialkyldiphenylamine compounds; e.g.,4,4′-dibutyldiphenylamine, 4,4′-dipentyldiphenylamine,4,4′-dihexyldiphenylamine, 4,4′-diheptyldiphenylamine,4,4′-dioctyldiphenylamine, and 4,4′-dinonyldiphenylamine,polyalkyldiphenylamine compounds; e.g., tetrabutyldiphenylamine,tetrahexyldiphenylamine, tetraoctyldiphenylamine, andtetranonyldiphenylamine, and naphthylamine compounds; e.g.,α-naphthylamine, phenyl-α-naphthylamine, butylphenyl-α-naphthylamine,pentylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine,heptylphenyl-α-naphthylamine, octylphenyl-α-naphthylamine, andnonylphenyl-α-naphthylamine; and sulfur-containing antioxidants such asthioterpene compounds; e.g.,2,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamino)phenoland a reaction product between phosphorus pentasulfide and pinene, anddialkyl thiodipropionates; e.g., dilauryl thiodipropionate and distearylthiodipropionate.

Examples of the rust preventive include metal sulfonates and succinateesters. Examples of the metal deactivator include benzotriazole andthiadiazole.

Examples of the viscosity index improver include polymethacrylates,dispersed polymethacrylates, olefin copolymers (e.g., ethylene-propylenecopolymer), dispersed olefin copolymers, and styrene copolymers such as(e.g., styrene-diene hydrogenated copolymer).

Examples of the pour point depressant which may be used in the inventioninclude polymethacrylates having a mass average molecular weight ofabout 50,000 to about 150,000.

The defoaming agent is preferably a silicone polymer-based defoamingagent. Through incorporation of the silicone polymer-based defoamingagent, defoaming performance can be effectively attained, whereby ridingcomfort can be improved.

Examples of the silicone polymer-based defoaming agent includeorganopolysiloxanes. Among them, fluorine-containing organopolysiloxanessuch as trifluoropropylmethylsilicone oil are particularly preferred.

The lubricating oil composition of the present invention may be appliedto any of a multi-cylinder shock absorber and a single-cylinder shockabsorber, and shock absorbers of a four-wheeled vehicle or a two-wheeledvehicle. The composition of the present invention is particularlysuitably used in four-wheeled vehicles.

Particularly, the lubricating oil composition of the present inventionlowers friction coefficient at the interface between bronze and chromiumand reduces wear area of bronze. Thus, the composition of the presentinvention is suitably used as a lubricant for a shock absorber which hasa guide bush at least including a bronze surface, and a piston rod atleast including a sliding part which comes into contact with the guidebush and which is made of chromium (e.g., chromium plating).

EXAMPLES

The present invention will next be described in more detail by way ofexamples, which should not be construed as limiting the inventionthereto.

Measurement of friction coefficient and a wear test were carried outthrough the following procedures.

(1) Measurement of bronze-chromium friction coefficient

Tester: Bowden-type reciprocating kinetic friction tester

Test conditions:

Load: 0.5 kgf

Stroke: 10 mm

Speed: 0.2 mm/s

Temperature: 80° C.

Friction operation: once

Friction members

Upper friction member: phosphor bronze ball

Lower friction member: chromium-plated sheet

(50×1,000×5 mm)

In a specific procedure, a ½-inch phosphor bronze was used. An oilcomposition sample was supplied to a plate, in an amount correspondingto several drops. The ball was conditioned on the plate (8 mm/s, 0.1 kgffor 2 minutes, 0.2 kgf for 2 minutes, 0.3 kgf for 2 minutes, and 0.5 kgffor 2 minutes). Thereafter, the wear test was performed at 0.2 mm/s.

(2) Wear test of bronze surface (wear area)

Tester: Bowden-type reciprocating kinetic friction tester

Test conditions:

Load: 0.5 kgf

Stroke: 10 mm

Speed: 8.0 mm/s

Temperature: 80° C.

Friction time: 30 minutes

Friction members

Upper friction member: phosphor bronze ball

Lower friction member: chromium-plated sheet

(50×1,000×5 mm)

In the wear test, a ½-inch phosphor bronze was used.

An oil composition sample was supplied to a plate, in an amountcorresponding to several drops. The wear area of the phosphor bronzeball was measured.

Examples 1 to 6, and Comparative Examples 1 to 4

Lubricating oil compositions (shock absorber oils) containing thecomponents given in Table 1 were prepared. Each composition wassubjected to the friction coefficient measurement and the wear test.Table 1 shows the results.

TABLE 1 Examples/ Comp. Comp. Comp. Comp. Comparative Examples Ex. 1 Ex.2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Compo- (A) Base oil99.50 90.00 99.50 99.90 99.50 95.00 99.50 99.50 99.50 99.50 nent (B) + αPentaerythritol dioleate 0.50 0.50 0.10 5.00 (mass %) Pentaerythritoldilaurylate 0.50 Pentaerythritol monooleate 0.50 Pentaerythritoltetraoleate 0.50 Stearic acid 0.50 Methyl acid phosphate 0.50 amine salt(C) Zn-DTP (pri) C8-C10 0.50 0.50 Test results Friction coefficient0.093 0.105 0.097 0.112 0.115 0.096 0.362 0.084 0.184 0.179 Wear area(mm²) 0.176 0.124 0.188 0.203 0.241 0.263 0.560 0.550 0.468 0.193

The components used in the Examples and Comparative Examples shown inTable 1 are as follows.

Base oil: secondary hydro-reformed mineral oil (paraffin-base) having akinematic viscosity of 7.83 mm²/s measured at 40° C.

Pentaerythritol dioleate: UNISTER H481D, product of NOF Corporation

Pentaerythritol monooleate: EKISUPARU PE-MO

Pentaerythritol tetraoleate: UNISTER H481R, product of NOF Corporation

Methyl acid phosphate amine salt: VANLUBE 672, product of Vandarbilt

Zn-DTP(pri): OLOA 5286, C8 to C10 mixed alkyl groups, product of EthylCorporation

According the oil compositions of the Examples, each containing (A) abase oil composed of a mineral oil and/or a synthetic oil and a specificamount of (B) a polyhydric alcohol partial ester having a C10 to C20fatty acid residue, friction coefficient and wear area were found to bereduced. Particularly in the case of the oil composition of Example 2,containing component (B) and component (C), wear area was considerablyreduced.

In Comparative Example 1, employing a complete ester of a polyhydricalcohol, friction coefficient and wear area were at high levels. InComparative Example 2, employing stearic acid, friction coefficient waslow, but wear area was large. In Comparative Example 3, employing ZnDTPinstead of component (B), friction coefficient and wear area were athigh levels. In Comparative Example 4, employing a phosphate amine salt,friction coefficient was higher than that obtained in the Examples.

INDUSTRIAL APPLICABILITY

The lubricating oil composition of the present invention can be used forautomobile shock absorbers. The composition is applicable to bothfour-wheeled vehicles and two-wheeled vehicles.

1. A lubricating oil composition for a shock absorber, comprising: (A) abase oil composed of a mineral oil and/or a synthetic oil, and; (B) apolyhydric alcohol partial ester having a C10 to C20 fatty acid residue,in an amount of 0.05 mass % to 10 mass %, with respect to the totalamount of the composition.
 2. A lubricating oil composition for a shockabsorber according to claim 1, wherein the component (B) is atetrahydric alcohol partial ester.
 3. A lubricating oil composition fora shock absorber according to claim 1, wherein the component (B) ispentaerythritol dioleate and/or pentaerythritol dilaurylate.
 4. Alubricating oil composition for a shock absorber according to claim 1,which further comprises (C) a phosphorus-containing compound.